There are several techniques employed for surface-mounting integrated circuits (ICs) onto printed circuit boards (PCBs). One common technique is the use of Ball Grid Arrays (BGAs) in which the pins of the IC are replaced by solder balls disposed on the underside of the IC package with the PCB carrying conductive pads in a pattern corresponding to the solder balls. In a technically-equivalent variation of this technique, the solder balls are formed instead on the PCB itself in a pattern that matches the pins of the IC.
Mounting the IC package to the PCB involves placing the IC in the correct position on the PCB and then heating the assembly, for example by means of an oven or infrared heater, causing the solder balls to melt in a process called “reflow”. During reflow, the IC sinks towards the PCB under gravity with the molten solder holding the package in alignment with the circuit board at the desired separation distance through surface tension. The solder then cools and solidifies to form the desired electrical contacts.
IC packages using a similar solder-ball technology are known as “flip-chips”, wherein the solder balls are deposited onto the top-side of the chip or wafer which is then flipped over and aligned with the matching pads on the PCB. The connections are again created by heating and melting the solder during the reflow process.
BGA packages have a number of advantages over conventional pin grid array (PGA) techniques and, as a result, are becoming widely used globally, particularly for larger PCBs with many connections since they allow for higher density components.
For example, BGA IC packages are easier to align to the PCB than using conventional metal connectors and relieve the problems associated with ICs having large numbers of pins with correspondingly decreased pin-to-pin spacing. In particular, as the distance between adjacent pins decreases, the incidence of pin shorting increases. BGAs generally do not have this problem when the solder is factory-applied to the package.
In addition, since the BGA connectors are located on the underside of IC, this has facilitated the provision of chip scale packaging (CSP) wherein the overall size of the IC package can be reduced to around 1.2× the size of the semiconductor die itself.
Furthermore, in comparison with IC packages having discrete leads, BGA's exhibit a lower thermal resistance between the package and the PCB, permitting heat generated by the circuit inside the package to flow more easily to the PCB, thereby reducing the occurrence of overheating.
Finally, with their very short distance between the package and the PCB, BGAs have lower inductance than IC packages having discrete leads. This reduces unwanted distortion of signals in high-speed electronic circuits and therefore provide superior electrical performance to conventional devices.
On the other hand, as with many electronic systems, the soldering process is not wholly reliable and open contacts or short circuits maybe formed inadvertently. In particular, since the solder balls are substantially non-compliant (that is, unable to flex), thermal or mechanical stresses caused, for example, by differences in coefficients of thermal expansion between the PCB substrate and the BGA package or by flexing and vibration of the PCB, may cause the solder joints to fracture.
It is therefore conventional to perform some form of testing of the IC connections on the PCB after soldering. In order to maintain production efficiency, an automated testing process is generally preferred. The automated inspection of electronic components is an integral part of electronic assembly manufacture to prevent and detect faults and to ensure quality and performance of the assembled systems. Increases in PCB complexity and the desire to improve yields has required the development of real-time automated inspection.
A problem with the use of BGAs, however, is that, because the connections are located on the underside of the IC, inspection of the soldered joints is very difficult. This may lead to faults not being diagnosed using standard automated inspection techniques, resulting in faulty products and a loss of revenue for manufacturers.
A variety of techniques have been proposed for testing the mounting of BGAs on PCBs. These techniques generally fall into two categories: physical inspection of the soldered connections; and electronic testing of the PCB circuit. Frequently, both physical and electronic tests are carried out in succession.
For example, visual inspection of the connections can be performed using Automated Optical Inspection (AOI) in which a video camera is used to scan the assembly and compare the scanned image with pre-recorded images of properly soldered PCBs in order to determine the efficacy of the soldered joints. However, due to the location of the solder connections, AOI systems are usually not able to measure the integrity of the solder joints and are generally limited to diagnosing missing components and placement errors.
An alternative technique is known as Automated X-ray Inspection (AXI) which is generally similar to AOI but employs an X-ray source instead of visible light. The penetrative nature of X-rays allows AXI devices to inspect features which are hidden from view, such as the solder connections beneath a BGA package, and to identify faults such as open circuits, short circuits, insufficient solder, missing parts and misaligned components.
In Circuit Testing (ICT) is also a method available for testing of electronic PCBs using an electrical probe that can check for shorts, opens, resistance, capacitance and other parameters to show if the PCB has been fabricated correctly. Typically ICT uses a “bed of nails” test feature or fixtureless in-circuit testing. ICT is typically used in conjunction with AOI or AXI.
Limitations of the above-mentioned automated inspection techniques for BGA assemblies can be summarised as follows:
1. The cost of AOI, AXI and combined AOI/AXI systems is prohibitively expensive for many production lines;
2. The connections between the IC and the PCB are located on the underside of the IC and thus hidden from optical inspection, making it difficult to inspect the quality of the solder joints;
3. AOI does not allow for the identification of open-circuits or short circuits;
4. Warping of ICs (which commonly occurs due to heating) is difficult to correct for using AOI and/or AXI;
5. AXI is relatively slow compared to line-speeds;
6. Using ICT, the quality of electrical contacts cannot be tested;
7. Achieving 100% ICT access cannot be cost-effectively achieved for BGAs due to the increased node count associated with BGA technology.
It is against this background that the present invention has been conceived. The present invention aims to address one or more of the above problems and to improve upon known techniques for testing the mounting of ICs to PCBs. Embodiments of the invention may provide a method and/or apparatus that enables the integrity of the soldered joints to be determined quickly, accurately and cost-effectively in an automated process. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.